1. Field of the Invention
The invention relates to a device for the automatic precorrection of non-linearities, cross modulation and/or intermodulation, in a power amplification chain. Its particular field is that of television transmitters and rebroadcasting transmitters, in which the signal to be transmitted is an amplitude-modulated signal. Thus, when power amplifiers have non-linearities, the amplitude modulation of the signal to be transmitted undergoes a distortion which is prejudicial to the quality of the signal received.
In television transmitters having two separate amplification chains, one used for the amplification of the picture video signal and the other for the amplification of the audio-frequency signal, the deficiencies resulting from the non-linearities of the amplifiers lead to an intermodulation of the amplified signal, i.e. a spurious signal is superimposed on the modulation signal to be transmitted. In the case of common channel television transmitters, i.e. in which the video frequency signal and the audio-frequency signal, both in amplitude-modulated form, are simultaneously transmitted and amplified, the non-linearities in the amplification chain on the one hand lead to an intermodulation spurious signal on each of the modulated carriers, and on the other hand to a spurious cross modulation signal due to the modulation transfer between the individual carriers.
The quality criteria for television transmitters or rebroadcasting transmitters are such that the noise, i.e. the spurious signals must be at a level of 60 decibels below the useful signal. However, when no correction is made, noise level is only 20 decibels below the useful signal. It is therefore absolutely indispensable to use non-linearity correction means in such systems.
2. Description of the Prior Art
French Pat. No. 2 275 065, corresponding Great Britian Pat. No. GB P 1 491 798 in the name of the present Applicant, described a correction circuit which is particularly suitable for the corrections of non-linearities of the third order in a transmitter with separate channels. In this circuit, the signals at the input and output of the amplification chain are detected, the signal at the chain output also being divided by a coefficient equal to the linear part of the amplification chain gain, said two signals being compared in a subtracter, which supplies an error signal at the angular frequency of the subcarrier to be corrected. This error signal is then added to the input signal in a precorrection circuit preceding the amplification stages. Such a precorrection device is very suitable for the correction of intermodulation, but does not make it possible to conveniently correct the distortion due to the cross modulation between individual carriers in a common channel transmitter. Thus, such a system makes it possible to appropriately correct a constant power subcarrier. However, when the subcarrier to be corrected is itself amplitude-modulated, and consequently has a variable instantaneous power, this correction circuit does not permit the correction thereof in its complete variation range.